1. Technical Field
The invention relates to broadcasting of data in digital form in an environment disturbed by noise and/or by the existence of multiple paths. It is particularly suitable for use in digital sound broadcasting toward movable receivers, since sound transmission makes it practically impossible to repeat sequences and consequently requires a high degree of immunity from the transmission system.
Difficulties in broadcasting towards vehicles--or communication of vehicles with each other--result from disturbing phenomena whose characteristics (frequency, amplitude, duration, . . . ) vary greatly depending on the origin of the phenomena, on the frequency band used and the location of the vehicle carrying the receiver in the reception zone.
2. Prior Art
Numerous methods are already known for limiting reception disturbances:
repeating the transmitted data and detecting (and possibly correcting) errors on arrival, which limits the effect of pulsed noise, even if it has a wide band;
coding the data using a convolutive code providing redundancy which increases the distance between transmitted signals and reduces the number of errors on reception;
increasing the duration of each of the symbols transmitted, which has a favorable as regards all disturbances but which limits the maximum rate, if a single frequency is used or requires frequency multiplexing (U.S. Pat. No. 2,705,795);
so called "frequency jump" broadcasting, consisting in transmitting the successive symbols alternately on several frequencies, only one of which is used at a time, which reduces the influence of echos and recurrent jamming, because they only act on a limited frequency range, while they may be continuous;
data interlacing which reduces the influence of disturbances of limited duration.
None of these solutions is completely satisfactory in a reception site affected by disturbances of different kinds, corresponding to different parts of the time-frequency space. That situation typically exists when broadcasting in a city environment, where there are numerous radio electric echos and disturbances of home or industrial origin.